Over the past few years, the cordless, battery operated powered surgical tool has become a very popular tool for use in surgery. As the name implies, this type of tool is provided with a battery that serves as the power source for the energy-consuming component integral with the tool. Typically, this component is an electrically driven motor. The integration of the battery into the tool eliminates the need to provide the tool with a power cord that is connected to an external power source. The elimination of the power cord offers several benefits over corded surgical tools. Surgical personnel using this type of tool do not have to concern themselves with either sterilizing a cord so that it can be brought into the sterile surgical field surrounding the patient or ensuring that, during surgery, an unsterilized cord is not inadvertently introduced into the surgical field. Moreover, the elimination of the cord results in the like elimination of the physical clutter and field-of-view blockage the cord otherwise brings to a surgical procedure.
An integral part of any battery-powered tool is, naturally, the battery. Most battery-powered surgical tools are designed to be used with rechargeable batteries. These rechargeable batteries, like other rechargeable batteries, typically are formed from one or more NiCd cells. Once a battery is discharged, it is coupled to a complementary charger. The charger applies a current to the battery's cells to store energy in the cells.
Unlike other rechargeable batteries, a rechargeable battery intended for use with a surgical tool must be sterilizable so that it can be placed in close proximity to the open surgical site on a patient. Often, these batteries are sterilized by placing them in an autoclave wherein the temperature is approximately 270.degree. F., the humidity is approximately 100% and the atmospheric pressure approximately 30 psi. The repetitive exposure to this environment causes a battery cells' ability to store electric charge to degrade.
Modern battery chargers, especially those that are used to charge sterilizable batteries, do more than simply apply currents to the batteries with which they are used. Often, a battery charger includes a circuit for measuring the voltage across a battery as it is being charged. A charging circuit, in turn, applies a selected current to the battery based on the voltage across the battery. Many chargers also include a load resistor that is selectively connected across the battery. Once the battery charging cycle has been completed, this resistor is connected across the battery and the voltage across it is measured. This voltage measurement is used to determine the state of the cells integral with the battery. This measure of cell state is, in turn, used as measure of the utility of a battery. For example, if, due to sterilization-induced degradation, the cells of a battery can no longer hold a charge, the voltage over the load resistor will be relatively low. If the load resistor voltage is below a select value, some chargers are configured to generate a display that indicates that the battery's current-storage capabilities have decayed to the point where the battery should be replaced.
While present battery chargers have proven to be useful devices, they are not without some limitations. It has been found that a voltage across a load resistor only provides a relatively inaccurate measure of the charge stored with a set of battery cells. Also, as the charge-storing capacity of a sterilizable battery decreases, present chargers do not offer any indication if such decrease is within a normal range.
Furthermore, present battery chargers are only designed to be used with one particular type of battery. There are two reasons for this. First, most battery chargers are provided with sockets designed to receive batteries having a specifically shaped head. Secondly, most battery charges are merely provided with components that are configured to deliver charge to one type of battery. Specifically, the internal circuitry of a typical charger is configured to deliver a select set of currents in response to a select set of battery voltage measurements. Consequently, each time a facility purchases a product that employs a new type of rechargeable battery, it is also required to purchase the charger that goes with the battery. The need to make this extra charger purchase, in addition to increasing the overall cost of the product purchase, requires the facility to store and maintain one additional piece of equipment.